Temperature-independent giant dielectric response in transitional BaTiO3 thin films

Abstract

Ferroelectric materials exhibit the largest dielectric permittivities and piezoelectric responses in nature, making them invaluable in applications from supercapacitors or sensors to actuators or electromechanical transducers. The origin of this behavior is their proximity to phase transitions. However, the largest possible responses are most often not utilized due to the impracticality of using temperature as a control parameter and to operate at phase transitions. This has motivated the design of solid solutions with morphotropic phase boundaries between different polar phases that are tuned by composition and that are weakly dependent on temperature. Thus far, the best piezoelectrics have been achieved in materials with intermediate (bridging or adaptive) phases. But so far, complex chemistry or an intricate microstructure has been required to achieve temperature-independent phase-transition boundaries. Here, we report such a temperature-independent bridging state in thin films of chemically simple BaTiO 3 . A coexistence among tetragonal, orthorhombic, and their bridging low-symmetry phases are shown to induce continuous vertical polarization rotation, which recreates a smear in-transition state and leads to a giant temperature-independent dielectric response. The current material contains a ferroelectric state that is distinct from those at morphotropic phase boundaries and cannot be considered as ferroelectric crystals. We believe that other materials can be engineered in a similar way to contain a ferroelectric state with gradual change of structure, forming a class of transitional ferroelectrics. Similar mechanisms could be utilized in other materials to design low-power ferroelectrics, piezoelectrics, dielectrics, or shape-memory alloys, as well as efficient electro-and magnetocalorics.

Beyond Substrates: Strain Engineering of Ferroelectric Membranes

“…[ 22,23 ] In thin films, strain can define the optimal operating temperature regime [ 24–26 ] or domain configuration [ 27–29 ] that will boost electro‐mechanical or thermal functionalities, whose associated susceptibilities (piezo‐/pyroelectricity) are enhanced at temperatures near phase transitions [ 30 ] or when competing microstructures coexist. [ 31–34 ] While strain is typically introduced by lattice mismatch with a substrate in epitaxial films, released films relax their lattice towards their bulk crystal structure; therefore, different approaches to manipulate properties are required. Mechanical and electric‐field manipulation of micrometer‐sized freestanding flakes in electron‐microscopy experiments [ 35,36 ] and straining experiments on polymer‐supported ultrathin perovskite membranes (restricted to films <10 nm in thickness) [ 37,38 ] have demonstrated their exceptional flexibility.…”

Domain-wall engineering and topological defects in ferroelectric and ferroelastic materials

“…Here, ionic, electronic, strain and exchange couplings at interfaces result in transition regions, often down to a couple of atomic layers only, both in the ferroelectric and ferromagnetic materials [167][168][169][170][171][172][173][174] . Symmetry breakings have also been observed in homogeneous thick films 175,176 . In this regard, the recent discussion in a work of Noheda on so-called "transitional ferroelectrics" is appealing 176 .…”

“…Symmetry breakings have also been observed in homogeneous thick films 175,176 . In this regard, the recent discussion in a work of Noheda on so-called "transitional ferroelectrics" is appealing 176 . The denomination comes from the observation of a gradual change of structure from a tetragonal symmetry at the top of a thick film of BaTiO3 to an orthorhombic symmetry at the bottom 176 .…”

“…In this regard, the recent discussion in a work of Noheda on so-called "transitional ferroelectrics" is appealing 176 . The denomination comes from the observation of a gradual change of structure from a tetragonal symmetry at the top of a thick film of BaTiO3 to an orthorhombic symmetry at the bottom 176 . The transition region between both phases is characterized by a reduced symmetry where the polarisation rotation is coupled to the strain gradient (FIG.…”

Ferroelectric Gating of Narrow Band-Gap Nanocrystal Arrays with Enhanced Light–Matter Coupling

“…This result is even more promising given the recent developments relative to the growth of thin films of ferroelectric oxides. [52][53][54] Such developments may lead to capacitances as high as the one of electrolyte without ion displacement. This makes this method highly promising for the design of FETs and phototransistors operated in cryogenic conditions.…”